Katherine looks at the wound, which looks like it would have been pretty nasty to endure, but also like it's getting much better. What draws her attention more is the scent of blood that still comes from it - it's not human, obviously, but there's something about it that smells delicious, irresistible, and suddenly there's a flash of red in her vision that, to Ilde, may make it look as if her eyes have gone black. She's about to say something when Ilde tells her to look behind her, and then she sees it - a monkey-like creature, one of the parasitic clingers to the larger, clumsier thing that's been lumbering through the city.

She doesn't hesitate, grabbing a nearby rock and heading straight for it - it's a primitive weapon, but she's had to use worse. The worry of infection doesn't bother her, she'll heal from nearly any wound inflicted on her (aside from a werewolf's bite, but this thing isn't a werewolf) and so she's able to get close, killing the hideous thing with one well-aimed blow to the skull.

Then she turns back to Ilde, and if it wasn't obvious what she is before it's crystal clear now - her face is fully turned, eyes gone nearly black, fangs extended, and veins protruding from the skin of her face. She doesn't need to turn her face to exhibit her full strength, it's more of a warning - this is what I can do, I'm capable of much worse, and I won't hesitate to show you how much worse if you cross me.

It's also a trade-off, an eye-for-an-eye - Ilde's displayed what she is, Katherine feels it only fair to do the same. Also, she hasn't survived for over five centuries by closing herself off to people, preventing herself from making contacts. Ilde is someone who could prove to be very valuable to know, and aside from that, she genuinely likes her, based on the things she knows.